Metal food and beverage containers, such as cans and lidded trays, are usually coated on the inside to prevent reaction between the contents of the container and the metal. Such reactions can lead to both unwanted deterioration of the metal container and to damaging effects on the contents of the container, particularly in terms of changes in quality and taste.
Coating compositions for the interior of metal food and beverage containers are often applied to flat metal by roller coating before the flat metal is formed into the container. The coating is dried and/or cured before the metal is shaped into the form of a container. The containers are usually formed by a drawing process before being filled with food or a beverage. The coating compositions need to have very good flexibility, adhesion, sterilization resistance, stability properties and blush resistance. Flexibility and adhesion are very important if the coating composition is to remain intact during the container formation process when the coated metal is shaped into a container.
When the containers are filled with food or a beverage, the contents of the container are usually sterilized by heating the sealed container to a temperature of 120° C. to 140° C. for 10 to 90 minutes. The cured coating is then in direct contact with the contents of the container for a considerable period of time, which during storage can be for many years. During sterilization and storage, the coating is required to maintain its integrity to prevent corrosion of the metal and to prevent migration of the metal into the contents of the container. The coating must also not impair the contents of the container by releasing unwanted material or by altering the flavor or appearance of the contents. These properties impact not only the shelf life of the product but also public health and safety. Thus, there are particularly stringent and specific requirements for coating compositions for the interior of containers that hold food and beverages.
In some applications, coating compositions are used to bond the lid to the body of the container. For example, pet food containers can be provided in a tray with a thin, full length aluminum lid. The lid is attached to the tray by means of a coating composition, usually crosslinked, which on the application of heat and pressure forms an adhesive bond or seal between the lid and the tray. The contents of the container are accessed by simply peeling back the flexible aluminum lid which in turn breaks the seal and the coating composition. This is known as an easy-open end of a container. The strength of the seal is very important. It must be strong enough to survive the manufacturing and filling processes, yet not be so strong that breaking the seal becomes difficult.
In many applications, coating compositions that contact food or beverages are applied as very thin films to produce dried films of no more than 10 μm. In such circumstances, the coating compositions must be free of particulate matter greater than this size. Polymer dispersions are often used to formulate such coating compositions, but unfortunately polymer dispersions sometimes contain particles too large to be suitable for very thin films. Also, coating compositions derived from polymer dispersions are sometimes uneven with rough surfaces and poor adhesion and protective properties.
A further overall desire in the field is to reduce the number of coating layers and to ideally use monolayer coatings. If the desired properties for a particular application can be achieved with a single coating layer, this has preference over multilayer coating systems as the former results in a faster and simpler coating process and lower costs.